Process of preserving vegetable tanned leather



Patented May 24, "1949 PROCESS OF PRESERVING VEGETABLE TANNED LEATHER Arthur Cheshire, Cottingham, England, assignor to Richard Hodgson and Sons Limited, Beverley, England, a company of Great Britain No Drawing. Application December 12, 1945, Se-

rial No. 634,611. In Great Britain January 13,

2 Claims. 1

This invention relates to improvements in the treatment of vegetable-tanned leathers.

One object of the invention is to render vegetable-tanned leather less liable to deterioration from ageing. Another object is to render vegetable-tanned leather less susceptible to the detrimental effect of iron. Another object is to render vegetable-tanned leather less susceptible to the detrimental efiect of sulphuric acid. Another object is to render vegetable-tanned leather more stable against the action of oxidation or I light.

Leather for bookbinding, upholstery, boot-mak- We have now found that vegetable-tanned ileather can be rendered less liable to deteriora- :tion by forming therein by double decomposition .between an aqueous solution of a calcium salt and :an aqueous solution of a substance selected from the group consisting of oxalic acid, water-soluble oxalates and mixtures thereof, from about A; to about 15%, and preferably from about 5% to about of calcium oxalate.

Since free sulphuric acid is normally taken up by the leather from the atmosphere, this sulphuric acid will liberate free oxalic acid from the calcium oxalate, and the free oxalic acid so liberated will form an oxalate complex with any iron subsequently taken up. It has been found that the take up of iron by the leather is not serious in absence of sulphuric acid. The oxalic acid so liberated will also serve to protect the leather from ageing eflects of active oxygen liable to be formed in leather by the action of ultra-violet light or from per-sulphuric acid. Whatever may be the agencies at work during the ageing of leather, they are of an oxidative character and their ageing effect is substantially reduced by having free oxalic acid in the leather. It is known that the iron present in most leathers can be converted into an oxalate complex by treating the leather 'with free oxalic acid. Thus it is well known that :oxalic acid will remove iron stains from leather. It has now been found that such treatment of the leather with free oxalic acid to convert any iron therein into a soluble complex improves the resistance of the leather to ageing. It has been further found, however, that any free oxalic acid which may remain after such complex formation has taken place, does not effectively protect the leather from ageing over a substantial period of time. Any free oxalic acid remaining tends to be lost by oxidation or other means. Moreover, iron and sulphuric acid are subsequently taken 2 up by the leather. So far as we are aware, it has not been known hitherto to provide a water-insoluble oxalate in leather.

Preferably the leather is treated with free oxalic acid to convert any iron present therein into an oxalate complex before forming the calcium oxalate therein.

The invention includes the process for rendering vegetable-tanned leather less liable to deterioration from ageing which comprises converting any iron present therein into an oxalate complex and then forming calcium oxalate therein.

The amount of calcium oxalate required to be incorporated into the leather according to the invention to render it less liable to deterioration from ageing is not critical. For example, from one half of 1% to 15% of calcium oxalate is suitable, the preferred range being from 5% to 10%. The degree of protection from ageing conferred by any specific amount of calcium oxalate depends upon a number of factors including the use to which the leather is put and its subsequent processing.

Leather treated according to the invention resists the attack of sulphuric acid and hydrogen peroxide in the concentrations of the former which have been observed in aged leathers.

Leather containing calcium oxalate resists the destructive effect of burnt coal gas fumes as produced in the so-called gas chamber test to a greater degree than leather not containing calcium oxalate. It may also be treated with iron to the extent of 200 parts per million without appreciable increase in the rate of degradation.

Leather containing calcium oxalate may be washed with water at elevated temperatures or with weakly alkaline liquors such as borax solution and dyed in weakly acid baths without the loss of the protective agent.

Calcium oxalate which can easily be formed in situ in the leather by treating the leather with an aqueous solution of oxalic acid or of a watersoluble oxalate or of both and then treating it with an aqueous solution of a water-soluble calcium salt such as calcium acetate.

Example 1.Cow-hide is limed and unhaired by the usual commercial methods and split. The grain split is delimed by ammonium chloride solution, washed with a solution containing oxalic acid so as to produce a pH of 2.5 and sufiicient salt (5% NaCl) to prevent swelling. The pelt is brought to pH 4.0 by addition of a weak base (borax) to a fresh salt solution of the same strength and tanned with mimosa extract of strength increasing from 2% to 10% containing enough oxalic acid to keep the pH at 4.0. The tanned leather is Washed once with water and dried roughly. It is then immersed in a 10% solution of oxalic acid (saturated) at 20 0., until soaked, transferred to excess of calcium acetate solution until the wholeof the oxalic acid is converted .to .calcium oxalate. t -After fat-liquoring the leather is dried.

Example 2.--A dry commercial vegetable tanned leather is Washed in the paddle or drum with water containing a soluble oxalate and oxalic acid at a pH 2.5 and the washings are discarded. Without further addition .of ,water, potassium oxalate is drummed into theleather (2 (COOK) 2) J which comprises treating vegetable tanned leather with an aqueous solutionof a calcium salt and an aqueoussolution of-a substanceselected from 'the group consisting of oxalic acid, water-soluble -oxalates and mixturesthereof to precipitate from about /2% to about 15% of calcium oxalate therein.

REFERENCES CITED iihe followingmeferences are of record in the fileof this patent:

.UNITED STATES PATENTS S ientific Librar o "Raten 91119. 

